The Temperature Dependence of Biological Rates from Enzymes to Ecosystems
Monday, 15 December 2014
Can enzymology and thermodynamics shed light on the response of the biosphere to a changing climate? We have recently developed a theory describing the temperature dependence of biological rates. We have called this MacroMolecular Rate Theory (MMRT) to reflect some fundamental thermodynamic properties peculiar to biological macromolecules. This theory scales well from enzymes to ecosystems and explains the curved temperature dependence of ecosystem processes such as respiration, as described by Lloyd and Taylor 20 years ago. MMRT also accounts for temperature optima which are a feature of all biological processes including respiration, photosynthsis and net ecosystem exchange. MMRT begins with enzymes. Enzymes drive metabolism and enable life by catalysing a myraid of chemical reactions with phenomenal rate enhancements. According to the classical thermodynamics description, enzymes achieve catalysis by binding to the transition state for the reaction and thus, lowering the reaction barrier. The dissociation constant, Kd, for the enzyme-transition state complex, commensurate with the observed rate enhancements, is extreme (Kd ~ 10-22 M). Such tight binding of the transition state influences the thermodynamic parameter, Cp, the heat capacity of the molecule. The difference in heat capacity, ∆C‡P, between the enzyme-substrate complex (Kd ~ 10-5 M) and the enzyme-transition state complex (Kd ~ 10-22 M) has important implications for the temperature dependence of enzyme catalyzed rates. ∆C‡P is close to zero for reactions that involve small molecules, but is generally large and negative for reactions that involve macromolecules such as enzymes. The result is a curved temperature dependence of enzyme catalyzed rates and a temperature optimum above which, the rate decreases. This ∆C‡P signature is pervasive and scales from enzymes, to microbial growth rates, to microbial metabolism and ecosystem fluxes. It also has important implications for the temperature sensitivity of all biologically driven processes and the response of the biosphere to climate change.